1. Field of Endeavor
The invention deals with the field of materials science. It relates to a steel-based welding additive material which is distinguished by improved meltability during welding and by a higher creep rupture strength after solidification than known welding additive materials.
2. Brief Description of the Related Art
It is known to produce rotors of thermal turbomachines, for example gas turbines, from individual disks which are then welded to each other. By way of example, this has been carried out by ABB, and then ALSTOM, for decades using an arc fusion welding process/submerged arc welding process.
The efficiency of gas turbines is increased, inter alia, by operating them at extremely high temperatures. Therefore, the rotors have to have both a high creep rupture strength at very high temperatures and also good further mechanical properties and a good oxidation resistance. It is self-evident that this also applies to the weld seams via which the rotor disks are connected to each other.
The use of a welding additive material having the following chemical composition (amounts in % by weight) is known from the prior art for the submerged arc welding of such gas turbine rotors:
0.09-0.14 C, max. 0.40 S, max. 1.40 Mn, max. 0.025 P, max. 0.020 S, max. 11.00-12.50 Cr, 2.00-2.60 Ni, 0.95-1.80 Mo, 0.20-0.35 V, 0.020-0.055 N, remainder iron.
This welding additive material is known by the name SZW 3001-UP. It is supplied as a wire, where the tensile strength of the wire is in the range from 700 to 1200 N/mm2 and the permissible deviation of the strength within a batch may not be more than +/−50 N/mm2. According to delivery conditions, this welding additive material is used for submerged arc joining welds and build-up welds.
However, this material no longer always satisfies the high demands of modern gas turbines, in particular with regard to high-temperature properties such as, for example, the creep rupture strength.
U.S. Pat. No. 5,906,791 discloses a boron and rare earth metal steel which is used for producing turbine components. It has very good physical and thermal properties, for example a high resistance to embrittlement, oxidation, and creep. In addition to the high chromium content (8-13% by weight Cr), this high-alloy steel also contains, inter alia, 0.001-0.04% by weight B, 0.01-2.00% by weight of at least one of the elements Re and Os, Ir, Ru, Rh, Pt, Pa (noble metals of the platinum group), and also 0.01-0.50% by weight of at least one rare earth metal, e.g., Y, La, C. In order to obtain a good toughness of the steel, it is desirable for this steel to contain, by way of example, as far as possible no more than 0.01% by weight Si and 0.05% by weight Mn, since this reduces the formation of segregations and second phases. In the indicated range, the rare earth metals reduce the aging embrittlement, while some, e.g., La, also reduce the formation of depositions. A prerequisite for this is a low level of impurities. Re and these metals of the platinum group act as elements for strengthening the solid solution. The latter additionally improve the oxidation resistance but disadvantageously increase the material costs.